When silicon is subjected to directional solidification in moulds e.g. graphite moulds, the moulds would have to be coated in order to avoid contamination of the silicon ingot from contact with the mould material. It is also important that the silicon ingot is easily removed from the mould after solidification without damaging or even destroying the mould. The moulds are therefore coated, generally with silicon nitride particles, which may be sprayed, painted or otherwise applied, in the form of a slurry, on to the inner surface of the moulds.
In the case of graphite moulds, it has been observed that they often break during solidification of the silicon ingot due to the fact that silicon is a material which expands on solidification from liquid. If a mould breaks during solidification, it cannot be reused. Furthermore, and more importantly, there may be leakage of liquid silicon during the solidification process which may destroy the furnace in which the mould is located during the process. For economical reasons it is very important that graphite moulds (and moulds made from other materials) can be reused many times.
U.S. Pat. No. 5,431,869 describes a mould treatment in which the mould is first coated with a Si3N4 powder and then an alkaline earth metal halide melt film is additionally formed between the silicon nitride powder coating and the silicon melt. This procedure is, however, costly, and contaminates the silicon melt in an undesirable way with alkaline earth metal and, possibly, with other impurities from the alkaline earth metal halide mixture used.
U.S. Pat. No. 6,615,425 describes a process in which a mould is coated with Si3N4 powder having a particular particle aspect ratio and oxygen content. It may be applied as an aqueous dispersion. However, the performance is not entirely satisfactory in that the coating is sometimes porous and allows infiltration of liquid metal in the coating.
It is therefore an object of the invention to provide a coating system for a mould which more reliably prevents damage to the mould material while at the same time enabling release of a silicon ingot from the mould without damage to the mould.